1. Field of the Invention
The present invention relates to a multilayer wiring board having through-holes of small diameter, wherein the through-holes filled with hardened filling material, and openings of the through-holes are closed with lid conductors.
2. Description of the Related Art
Along with the recent trend in the art to smaller size and higher performance electronic appliances, there is the demand that the mounting density of electronic components be higher. To achieve such high density mounting, the use of the multilayer forming technology for wiring boards is important. A specific example of this multilayer forming technology is known as a multilayer wiring board, in which interlayer insulating layers and wiring layers are alternately laminated to form a built-up layer on one side or both sides of a core substrate having through-holes of about 300 μm in diameter.
When manufacturing such multilayer wiring board, in order to assure a flat built-up layer, through-holes must be preliminarily filled up with a filler material, such as a through-hole filling paste, which should be heated and cured. Several methods have been proposed for filling up through-holes (see, for example, Japanese Laid-open Patent Publication No. H10-75027 and Japanese Laid-open Patent Publication No. H11-199759). The first reference discloses a technique for preventing cracks in the interlayer insulating layer by filling through-holes using a resin filling material specified as to the resin component and type of curing agent. The second reference discloses a technique for preventing cracks in the interlayer insulating layer by filling through-holes using a filling material having a characteristic shrinkage rate of a hardened material.